Recently, from the viewpoint of environmental protection and pollution control, there is a demand for reducing waste by promoting the use of recycled resources. In particular, since plastics are used for various applications of use, for example, household electrical appliances, automobiles, construction materials, etc., it is very important to promote the use of plastics as recycled resources.
In order to promote the recycling of plastics, firstly, it is necessary to distinguish the kind of waste plastics. Conventionally, as a method for distinguishing a kind of plastics, a near infrared spectroscopy and an infrared spectroscopy have been employed because of their abilities for highly precise analysis. An example of a general technique for such a spectroscopy includes a dispersed reflection method and a transmission method. However, with such techniques, if the plastic to be tested is black, all of near infrared rays and infrared rays are absorbed by plastics, so that necessary spectra cannot be obtained. Consequently, it was impossible to distinguish plastics.
As a method for distinguishing such plastics, the attenuated total reflection spectroscopy (hereinafter “ATR” will be referred to) is known. In this method, a sample having a low refractive index is brought into close contact with a crystal having a high refractive index; infrared rays are allowed to be incident in the interface between the crystal and the sample at an angle that is larger than the angle for causing a total reflection (critical angle); and the incident infrared rays are allowed to be reflected at the interface, and thereafter the reflected spectrum is measured.
By employing this ATR, every plastic material including a black-color sample can be distinguished with high precision. However, if the ATR is employed, in order to bring the sample into contact with the crystal, it was necessary to carry out sampling so that the sample has substantially the same area as the area of the surface of the crystal (for example, about 5 mm×about 20 mm) and the sample can be brought into contact with the entire crystal, and also necessary to shave the entire surface of the test piece so that an error in distinguishing is not caused by the influence of impurity or a surface treatment agent (for example, coating, plating, etc.) attached to the surface of the test piece. Thus, conventionally, in order to carry out the measurement by the ART, the test piece having a relatively large area was required. Furthermore, it was necessary to shave the entire surface of the test piece, and thus it took much time and efforts to prepare test pieces. In particular, for the purpose of recycling, continuous processing of a large amount of plastic materials is required, and thus the plastic materials are required to be distinguished simply and speedily. Therefore, the above-mentioned conventional method in which the preparation and pretreatment of test piece are required was not practical.